Non-volatile memory device having vertical structure and method of operating the same

ABSTRACT

A non-volatile memory device having a vertical structure includes a NAND string having a vertical structure. The NAND string includes a plurality of memory cells, and at least one pair of first selection transistors arranged to be adjacent to a first end of the plurality of memory cells. A plurality of word lines are coupled to the plurality of memory cells of the NAND string. A first selection line is commonly connected to the at least one pair of first selection transistors of the NAND string.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/094,184 filed on Apr. 8, 2016, which is acontinuation application of U.S. patent application Ser. No. 14/302,025filed on Jun. 11, 2014, now U.S. Pat. No. 9,336,884, issued May 10,2016, which is a continuation application of U.S. patent applicationSer. No. 13/658,461 filed on Oct. 23, 2012, which is a divisionalapplication of U.S. patent application Ser. No. 12/658,072, filed onFeb. 2, 2010, now U.S. Pat. No. 8,295,089, issued Oct. 23, 2012, whichclaims the benefit of Korean Patent Application No. 10-2009-0008041,filed on Feb. 2, 2009 and Korean Patent Application No. 10-2009-0083148,filed on Sep. 3, 2009, in the Korean Intellectual Property Office, thecontents of which applications are incorporated herein in their entiretyby reference. This application may also be related to U.S. patentapplication Ser. No. 13/836,212, filed Mar. 15, 2013, which is acontinuation application of U.S. patent application Ser. No. 12/860,049,filed on Aug. 20, 2010, now U.S. Pat. No. 8,488,381, issued Jul. 16,2013, which is a continuation-in-part application of U.S. patentapplication Ser. No. 12/658,072, filed on Feb. 2, 2010, now U.S. Pat.No. 8,295,089, issued Oct. 23, 2012, which also claimed the benefit ofKorean Patent Application No. 10-2009-0008041, filed on Feb. 2, 2009 andKorean Patent Application No. 10-2009-0083148, filed on Sep. 3, 2009, inthe Korean Intellectual Property Office.

BACKGROUND

The inventive concept relates to a semiconductor device, and moreparticularly, to a non-volatile memory device having a verticalstructure and a method of operating the same.

Although electronic devices have become continually reduced in size,they are nevertheless required to process a large amount of data. Thus,in order to reduce size while maintaining or improving upon processingcapabilities, non-volatile memory devices for use in such electronicdevices need to be reduced in size while increasing the integrationdegree thereof. To this end, non-volatile memory devices having avertical structure have been considered instead of those having aconventional flat structure. However, non-volatile memory devices havinga vertical structure are complicated to manufacture, and thus, thereliability thereof tends to be lower than more conventional memorydevices having a flat structure.

SUMMARY

In accordance with aspects of the present invention, provided is anon-volatile memory device having a vertical structure, with increasedreliability, and a method of operating the same.

According to an aspect of the inventive concept, there is provided anon-volatile memory device having a vertical structure. The non-volatilememory device includes a substrate; a semiconductor pole formed on thesubstrate to extend vertically with respect to the substrate; a NANDstring formed on the substrate to extend vertically with respect to thesubstrate and along sidewalls of the semiconductor pole, the NAND stringincluding a plurality of memory cells and at least one pair of firstselection transistors adjacent to a first end of the plurality of memorycells; a plurality of word lines coupled to the plurality of memorycells of the NAND string; and a first selection line being commonlycoupled to the at least one pair of first selection transistors of theNAND string.

In the non-volatile memory device, a bit line may be connected to afirst end of the NAND string, and a common source line may be connectedto a second end of the NAND string opposite to the bit line.

The at least one pair of first selection transistors may be arrangedbetween the bit line and the plurality of memory cells.

The at least one pair of first selection transistors may be arrangedbetween the common source line and the plurality of memory cells.

The NAND string may further include at least one pair of secondselection transistors that are adjacent to a second end of the pluralityof memory cells and opposite to the at least one pair of first selectiontransistors.

The non-volatile memory device may further include a second selectionline commonly connected to the at least one pair of second selectiontransistors of the NAND string.

According to another aspect of the inventive concept, there is provideda non-volatile memory device having a vertical structure. Thenon-volatile memory device including a substrate; a plurality ofsemiconductor poles formed on the substrate to extend vertically withrespect to the substrate, and arranged in a matrix; a plurality of NANDstrings formed on the substrate to extend vertically with respect to thesubstrate and along sidewalls of the semiconductor poles to form amatrix of rows and columns of NAND strings, the NAND strings eachincluding a plurality of memory cells and at least one pair of firstselection transistors adjacent to a first end of the plurality of memorycells; a plurality of bit lines, each bit line commonly connected to afirst end of each NAND string in a same row of NAND strings from theplurality of NAND strings; a common source line commonly connected to asecond end of each NAND string in the plurality of NAND strings, andopposite to the plurality of bit lines; a plurality of word lines, eachword line commonly coupled to memory cells arranged at a same level fromthe plurality of memory cells of the plurality of NAND strings; and aplurality of first selection lines, each first selection line commonlycoupled to the at least one pair of first selection transistors of eachNAND string in a same column of NAND strings from among the plurality ofNAND strings.

The plurality of memory cells may include control gate electrodes on thesidewalls of the plurality of semiconductor poles, and the at least onepair of first selection transistors may include first gate electrodes onthe sidewalls of the plurality of the semiconductor poles.

A difference between a gate length of each of the control gateelectrodes and a gate length of each of the first gate electrodes may bein a range of about 0 nm to 10 nm.

A gate length of each of the first gate electrodes may be less than orequal to the distance between first gate electrodes arranged in a samelevel between adjacent semiconductor poles.

A gate length of each of the control gate electrodes may be less than orequal to the distance between control gate electrodes arranged in a samelevel between adjacent semiconductor poles.

Each of the NAND strings can further include at least one pair of secondselection transistors adjacent to a second end of the plurality ofmemory cells and opposite to the at least one pair of first selectiontransistors.

The non-volatile memory device may further include a plurality of secondselection lines, each second selection line commonly coupled to the atleast one pair of second selection transistors of each NAND string in asame column of NAND strings from the plurality of NAND strings.

According to another aspect of the inventive concept, there is provideda method of operating a non-volatile memory device. The method includingapplying a program voltage to one memory cell selected from among aplurality of memory cells of a NAND string from a plurality ofvertically arranged NAND strings, and applying a pass voltage to theremaining memory cells of the NAND string, where the pass voltage isless than the program voltage; and applying a first voltage to a firstselection transistor closest to the plurality of memory cells in theNAND string, the first selection transistor from a pair of firstselection transistors that is adjacent to a first end of the pluralityof memory cells in the NAND string, and applying a second voltage to aremaining first selection transistor from the pair of first selectiontransistors, where the second voltage is less than the first voltage.

The first voltage may be substantially equal to the pass voltage.

The second voltage may be equal to or greater than a threshold voltageof the remaining first selection transistor.

The method may further include applying 0V to a pair of second selectiontransistors that is adjacent to a second end of the plurality of memorycells in the NAND string and opposite to the pair of first selectiontransistors.

The method may further include applying 0V to a bit line connected to afirst end of one of the plurality of NAND strings so that data stored ina memory cell selected from among the plurality of memory cells of theNAND string is programmed.

The method may further include applying an operational voltage to a bitline connected to one of the plurality of NAND strings to prevent datastored in the plurality of memory cells of the NAND string from beingprogrammed.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the inventive concept will be more clearlyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is an embodiment of a circuit diagram of anon-volatile memorydevice, according to aspects of the inventive concept;

FIG. 2 is another embodiment of a circuit diagram of a non-volatilememory device, according to aspects of the inventive concept;

FIG. 3 is a schematic cross-sectional view of the non-volatile memorydevice of FIG. 2, taken from a direction of word lines;

FIG. 4 is a schematic cross-sectional view of the non-volatile memorydevice of FIG. 2, taken from a direction of bit lines;

FIG. 5 is a graph showing the result of an experiment in which asaturated current flowing through selection transistors was measuredaccording to the distance between the selection transistors, accordingto an embodiment of the inventive concept;

FIGS. 6 through 12 are schematic cross-sectional views illustrating anembodiment of a method of fabricating a non-volatile memory device,according to aspects of the inventive concept;

FIG. 13 is another embodiment of a circuit diagram of a non-volatilememory device, according to aspects of the inventive concept;

FIG. 14 is a schematic cross-sectional view of the non-volatile memorydevice of FIG. 13, taken from a direction of bit lines;

FIG. 15 is a schematic block diagram of another embodiment of anon-volatile memory system, according to aspects of the inventiveconcept;

FIG. 16 is a schematic block diagram of an embodiment of a memory card,according to aspects of the inventive concept; and

FIG. 17 is a schematic block diagram of an embodiment of an electronicsystem, according to aspects of the inventive concept.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, exemplary embodiments of the inventive concept will bedescribed more fully with reference to the accompanying drawings. Theinventive concept may, however, be embodied in many different forms andshould not be construed as being limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill convey the inventive concept to those of ordinary skill in the art.In the drawings, the size of each element may be exaggerated forclarity.

The terms used in the following embodiments may be understood as beinggenerally known in the technical field to which the inventive conceptpertains. For example, the term, ‘at least one’ includes one or more ofthe associated listed items and is intended to include not only asingular form but also plural forms.

It will be understood that, although the terms first, second, etc. arebe used herein to describe various elements, these elements should notbe limited by these terms. These terms are used to distinguish oneelement from another, but not to imply a required sequence of elements.For example, a first element can be termed a second element, and,similarly, a second element can be termed a first element, withoutdeparting from the scope of the present invention. As used herein, theterm “and/or” includes any and all combinations of one or more of theassociated listed items.

It will be understood that when an element is referred to as being “on”or “connected” or “coupled” to another element, it can be directly on orconnected or coupled to the other element or intervening elements can bepresent. In contrast, when an element is referred to as being “directlyon” or “directly connected” or “directly coupled” to another element,there are no intervening elements present. Other words used to describethe relationship between elements should be interpreted in a likefashion (e.g., “between” versus “directly between,” “adjacent” versus“directly adjacent,” etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes” and/or “including,” when used herein, specifythe presence of stated features, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, steps, operations, elements, components, and/or groupsthereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper” and the like may be used to describe an element and/or feature'srelationship to another element(s) and/or feature(s) as, for example,illustrated in the figures. It will be understood that the spatiallyrelative terms are intended to encompass different orientations of thedevice in use and/or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” and/or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.The device may be otherwise oriented (e.g., rotated 90 degrees or atother orientations) and the spatially relative descriptors used hereininterpreted accordingly.

FIG. 1 is an embodiment of a circuit diagram of a non-volatile memorydevice according to aspects of the inventive concept. Referring to FIG.1, a NAND string NS may extend in a vertical direction, that is, it mayhave a vertical structure relative to a substrate (not shown). The NANDstring NS may have at least one pair of string selection transistors TS1and TS2, a plurality of memory cells MC, and at least one pair of groundselection transistors TG1 and TG2. A bit line BL may be connected to oneend of the NAND string NS and a common source line CSL may be connectedto the other end of the NAND string NS.

The memory cells MC may be arranged in series in the vertical direction.The memory cells MC may store data. A plurality of word lines WL0, WL1,through WLn−1, and WLn, where “n”+1 is the number of word lines, may berespectively coupled to the memory cells MC in order to control thememory cells MC. The total number of the memory cells MC may beappropriately determined according to the capacity of the non-volatilememory device.

The string selection transistors TS1 and TS2 may be arranged near oneend of the memory cells MC. For example, the string selectiontransistors TS1 and TS2 may be located between the bit line BL and thememory cells MC, and may be connected in series to the memory cells MC.The string selection transistors TS1 and TS2 may control an exchange ofsignals between the bit line BL and the memory cells MC. The stringselection line SSL may be commonly coupled to the string selectiontransistors TS1 and TS2. Thus, the string selection transistors TS1 andTS2 may operate together as if they were unified as one transistor.

The ground selection transistors TG1 and TG2 may be located opposite tothe string selection transistors TS1 and TS2, and closer to the otherend of the memory cells MC. For example, ground selection transistorsTG1, TG2 may be located between the common source line CSL and thememory cells MC, and may be connected in series to the memory cells MC.The ground selection transistors TG1 and TG2 may exchange signalsbetween the common source line CSL and the memory cells MC. The groundselection line GSL may be commonly coupled to the ground selectiontransistors TG1 and TG2. Thus, the ground selection transistors TG1 andTG2 may operate together as if they were unified as one transistor.

For example, in order to perform a programming operation, 0V may beapplied to the bit line BL, an ‘ON’ voltage (a turn-on voltage) may beapplied to the string selection line SSL, and an ‘OFF’ voltage (turn-offvoltage) may be applied to the ground selection line GSL. The ‘ON’voltage may be equal to or greater than a threshold voltage of thememory cells MC in order to turn on the string selection transistors TS1and TS2, and the ‘OFF’ voltage may be less than the threshold voltage inorder to turn off the ground selection transistors TG1 and TG2. Aprogram voltage may be applied to a memory cell MC selected from amongthe memory cells MC, and a pass voltage may be applied to the othermemory cells MC. When the program voltage is applied to the selectedmemory cell MC, electric charges may be injected into the selectedmemory cell MC due to Fowler-Nordheim (FN) tunneling. FN tunneling isknown in the art, so not discuss in detail herein. The pass voltage maybe greater than the threshold voltage.

In order to perform a read operation, a read voltage may be applied tothe bit line BL, and the ‘ON’ voltage may be applied to the stringselection line SSL and the ground selection line GSL. A referencevoltage may be applied to a memory cell MC selected from among thememory cells MC, and the pass voltage may be applied to the other memorycells.

In order to perform an erase operation, an erase voltage may be appliedto the bodies of the memory cells MC, and 0V may be applied to the wordlines WL0, WL1 through WLn−1, and WLn. Thus, data may be erased from thememory cells MC at the same time.

Alternatively, one of either the string selection transistors TS1 andTS2 or the ground selection transistors TG1 and TG2 may be omitted. Thestring selection transistors TS1 and TS2 and the ground selectiontransistors TG1 and TG2 may be respectively referred to as firstselection transistors and second selection transistors and vice versa.

FIG. 2 is another embodiment of a circuit diagram of a non-volatilememory device according to aspects of the inventive concept. Thenon-volatile memory device of FIG. 2 may correspond to an array of aplurality of non-volatile memory devices, such as that shown in FIG. 1.Thus, a description of the operations or characteristics of elementsthat are the same as those of FIG. 1 will not be provided here.

Referring to FIG. 2, a plurality of NAND strings NS11, NS12, NS21, andNS22, each having a vertical structure, may be arranged in a matrix. Afirst bit line BL1 may be commonly connected to the NAND strings NS11and NS21 in a first row, and a second bit line BL2 may be commonlyconnected to the NAND strings NS12 and NS22 in a second row. A commonsource line CSL may be commonly connected to be opposite to the firstand second bit lines BL1 and BL2 and may be commonly connected to theother ends of the NAND strings NS11, NS12, NS21, and NS22. The totalnumber of the NAND strings NS11, NS12, NS21, and NS22 and the totalnumber of the bit lines BL1 and BL2 are just examples thereof, and theinventive concept is not limited thereto.

Word lines WL0, WL1 through WLn−1, and WLn may be commonly connected tomemory cells MC arranged at the same level. A first string selectionline SSL1 may be commonly coupled to string selection transistors TS1and TS2 of the NAND strings NS11 and NS12 in the first row. A secondstring selection line SSL2 may be commonly coupled to string selectiontransistors TS1 and TS2 of the NAND strings NS21 and NS22 in the secondrow. A first ground selection line GSL1 may be commonly coupled toground selection transistors TG1 and TG2 of the NAND strings NS11 andNS12 in the first row. A second ground selection line GSL2 may becommonly coupled to ground selection transistors TG1, and TG2 of theNAND strings NS21 and NS22 in the second row.

In order to perform a program operation, 0V may be applied to a bit lineselected from the bit lines BL1 and BL2, and an ‘ON’ voltage may beapplied to the other bit line BL1 or BL2 for channel boosting. Also, the‘ON’ voltage may be applied to a string selection line selected from thestring selection lines SSL1 and SSL2 and an ‘OFF’ voltage may be appliedto the other string selection line SSL1 or SSL2. Thus, it is possible toselectively operate a NAND string that is commonly connected to theselected bit line and string selection line from among the NAND stringsNS11, NS12, NS21, and NS22.

In order to perform a read operation, a read voltage may be applied to abit line selected from the bit lines BL1 and BL2 and the other bit lineBL1 or BL2 may be floated. Also, the ‘ON’ voltage may be applied to astring selection line selected from string selection lines SSL1 and SSL2and the ‘OFF’ voltage may be applied to the other string selection lineSSL1 or SSL2. Thus, it is possible to selectively operate a NAND stringthat is commonly connected to the selected bit line and string selectionline from among NAND strings NS11, NS12, NS21, and NS22.

In order to perform an erase operation, an erase voltage may be appliedto the bodies of the memory cells MC and 0V may be applied to the wordlines WL0, WL1 through WLn−1, and WLn. Accordingly, data may be erasedfrom the memory cells MC of the NAND strings NS11, NS12, NS21, and NS22at the same time.

FIG. 3 is a schematic cross-sectional view of the non-volatile memorydevice of FIG. 2, taken from a direction of word lines. FIG. 4 is aschematic cross-sectional view of the non-volatile memory device of FIG.2, taken from a direction of bit lines.

Referring to FIGS. 2 to 4, a substrate 105 may be provided and prepared.The substrate 105 may be formed of a semiconductor material, e.g., a IVsemiconductor, a III-V compound semiconductor, or a II-VI oxidesemiconductor. For example, the IV semiconductor may include silicon,germanium, or silicon-germanium. In various embodiments, the substrate105 may be a bulk wafer or an epitaxial layer.

A plurality of semiconductor poles 130 may be formed to extend upwardlyfrom the substrate 105, here semiconductor poles 130 extend verticallyfrom the substrate 105. The NAND strings NS11, NS12, NS21, and NS22 maybe formed along and on the sidewalls of the semiconductor poles 130. Thesubstrate 105 may include an impurity-doped region 110 below thesemiconductor poles 130. The impurity-doped region 110 may be a sourceregion and may form a PN junction together with another region of thesubstrate 105. The common source line CSL of FIG. 2 may be connected tothe impurity-doped region 110.

Alternatively, a plurality of source regions may be formed only at thebottoms of the semiconductor poles 130, and the impurity-doped region110 may function as the common source line CSL. A plurality of drainregions may be formed only at the tops of the semiconductor poles 130and may be connected to the bit line BL1 of FIG. 2.

The memory cells MC may include a plurality of storage media 150 on thesidewalls of the semiconductor poles 130, and a plurality of controlgate electrodes 164 on the storage media 150. Each of the storage media150 may include a tunneling insulating layer 142 at least partiallyformed on a sidewall of one of the semiconductor poles 130, a chargestoring layer 144 formed on the tunneling insulating layer 142, and ablocking insulating layer 146 formed on the charge storing layer 144,and around the control gate electrode.

A PN junction source/drain region doped with impurities is not formedaround the surfaces of the semiconductor poles 130 between the controlgate electrodes 164. Thus, the semiconductor poles 130 in the memorycells MC may be continuously doped with impurities of the sameconductive type in order to form a well or a channel. In this case, thesemiconductor poles 130 between the memory cells MC may be electricallyconnected using a field effect source/drain region during a program/readoperation. The surfaces of the semiconductor poles 130 between thememory cells MC may be turned on using an electric field, i.e., afringing field, which is formed in the lateral direction of the controlgate electrodes 164.

The charge storing layers 144 may have charge storing capabilities. Thecharge storing layers 144 may be charge trapping layers, and mayinclude, for example, a silicon nitride layer, quantum dots, ornanocrystals. The quantum dots or nanocrystals may be formed of aconductive material, e.g., fine metal or semiconductor particles. Eachof the tunneling insulating layers 142 and the blocking insulatinglayers 146 may include an oxide layer, a nitride layer, or a high-Klayer. The high-K layer may be a dielectric layer, the dielectricconstant of which is greater than those of an oxide layer and a nitridelayer.

The string selection transistors TS1 and TS2 may include a plurality ofstring selection gate electrodes 166 on the sidewalls of thesemiconductor poles 130. The ground selection transistors TG1 and TG2may include a plurality of ground selection gate electrodes 162 on thesidewalls of the semiconductor poles 130. The string selection gateelectrodes 166 and the ground selection gate electrodes 162 may berespectively referred to as first selection gate electrodes and secondselection gate electrodes and vice versa.

A PN junction source/drain region doped with impurities is not formedaround the surfaces of the semiconductor poles 130 between the memorycells MC, and also is not formed among the string selection transistorsTS1 and TS2, the memory cells MC, and the ground selection transistorsTG1 and TG2. Instead, the semiconductor poles 130 between the memorycells MC may be electrically connected using the field effectsource/drain region during a program/read operation, as described above.

The storage media 150 between the string selection transistors TS1 andTS2 and the semiconductor poles 130 and between the ground selectiontransistors TG1 and TG2 and the semiconductor poles 130 may function asgate insulating layers and may be thus replaced with one insulatinglayer in some embodiments. A plurality of interlevel dielectric layers115 may be formed among the ground selection gate electrodes 162, thecontrol gate electrodes 164, and the string selection gate electrodes166. The storage media 150 may extend along the surfaces of theinterlevel dielectric layers 115.

The string selection gate electrodes 166 may be commonly connected tothe first string selection line SSL1 via contact plugs 174. The controlgate electrodes 164 may be respectively connected to the word lines WL0,WL1, through WLn−1, and WLn via contact plugs 172. The ground selectiongate electrodes 162 may be connected to the first ground selection lineGSL1 via contact plugs 170.

In the present embodiment, at least two string selection transistors,i.e., the string selection transistors TS1 and TS2, are used. Thus,since the gate lengths of the string selection gate electrodes 166 maybe still less than when only one string selection transistor is used, agap between the interlevel dielectric layers 115 may be filled with thestring selection gate electrode 166 without causing a void. Furthermore,at least two ground selection transistors, i.e., the ground selectiontransistors TG1 and TG2, are used. Thus, since the gate lengths of theground selection gate electrodes 162 may still be less than when onlyone ground selection transistor is used, a gap between the interleveldielectric layers 115 may be filled with the ground selection gateelectrode 164 without causing a void.

The gate length Ls of the string selection gate electrodes 166 may beless than or equal to the distance Ds between the string selection gateelectrodes 166 that face each other between adjacent semiconductor poles130. The gate length Lm of the control gate electrodes 164 may be lessthan or equal to the distance Dm between the control gate electrodes 164that face each other between adjacent semiconductor poles 130. The gatelength Lg of the ground selection gate electrodes 162 may be less thanor equal to the distance Dg between the ground selection gate electrodes162 that face each other between adjacent semiconductor poles 130.

The difference between the gate length Ls of the string selection gateelectrodes 166, the gate length Lm of the control gate electrodes 164,and the gate length Lg of the ground selection gate electrodes 162 maybe in a range of about 0 nm to 10 nm.

Accordingly, the gaps between the interlevel dielectric layers 115 in aNAND string (NS) may be filled with the string selection gate electrodes166, the control gate electrodes 164, and/or the ground selection gateelectrodes 162 without causing a void, thereby increasing the controlreliability of the string selection gate electrodes 166, the controlgate electrodes 164, and/or the ground selection gate electrodes 162.

FIG. 5 is a graph showing the result of an experiment in which asaturated current flowing through selection transistors was measuredaccording to the distance between the selection transistors, accordingto aspects of the inventive concept. The experiment of FIG. 5 shows thatthe greater the distance between the selection transistors, the less thesaturated current flowing through the selection transistors. However,the saturated current was not greatly changed. When the distance betweenthe selection transistors was 0 nm, it means that the selectiontransistors were formed in a single body. When the distance between theselection transistors was about 130 nm, the saturated current wasreduced by about less than 10% than the saturated current from when thedistance between the selection transistors was 0 nm. The reason why thesaturated current was not greatly changed even when the selectiontransistors were separated from each other is that no PN junctionsource/drain region is present between the string selection transistors.However, when the distance between the string selection transistorsincreases, the intensity of a fringing field may be weakened, thusgradually reducing the saturated current.

FIGS. 6 through 12 are schematic cross-sectional views illustrating anembodiment of a method of fabricating a non-volatile memory deviceaccording to aspects of the inventive concept. Referring to FIG. 6, animpurity-doped region 110 may be defined by implanting impurities intoan upper part of a substrate 105. Next, interlevel dielectric layers 115and sacrificial layers 120 may be alternately formed on the substrate105. The sacrificial layers 120 may be etched selectivity with respectto the interlevel dielectric layers 115. For example, interleveldielectric layers 115 may be oxide layers and the sacrificial layers 120may be nitride layers.

Referring to FIG. 7, a plurality of first holes 125 may be formed byetching the interlevel dielectric layers 115 and the sacrificial layers120. The first holes 125 may be formed using photolithography andetching processes, as examples. Next, a plurality of semiconductor poles130 may be formed to fill in the holes 125. For example, thesemiconductor poles 130 may be epitaxial layers having a polycrystallineor monocrystalline structure.

Referring to FIG. 8, a plurality of second holes 135 may be formed byetching the interlevel dielectric layers 115 and the sacrificial layers120 between the semiconductor poles 130. The second holes 135 may beobtained using the photolithography and etching processes. Within thepartial view of FIG. 8, only one second hole 135 is shown.

Referring to FIG. 9, the sacrificial layers 120 may be removed. Forexample, the sacrificial layers 120 may be removed by permeating anetchant between the interlevel dielectric layers 115 via the secondholes 135 using isotropic etching. For example, isotropic etching mayinclude wet etching or chemical dry etching, as examples. Thus, thesacrificial layers 120 between the interlevel dielectric layers 115 maybe removed to form a plurality of tunnels 140 connected to the secondholes 135. The tunnels 140 may expose portions of the sidewalls of thesemiconductor poles 130.

Referring to FIG. 10, a plurality of storage media 150 may be formed onthe sidewalls of the interlevel dielectric layers 115 and thesemiconductor poles 130, which are exposed via the second holes 135 ofFIG. 9 and the tunnels 140 of FIG. 9. The storage media 150 may beobtained by sequentially forming a tunneling insulating layer 142, acharge storing layer 144, and a blocking insulating layer 146. Next, aconductive layer 155 may be formed on the storage media 150. Forexample, the storage media 150 and the conductive layer 155 may beobtained using chemical vapor deposition or an electroplating methodhaving good step coverage.

A reactive source for forming the conductive layer 155 may be suppliedin a direction from the top of the second holes 135 to the bottomthereof. Thus, in order to form the conductive layer 155 in the tunnels140 of FIG. 4 without causing a void, the second holes 135 should not beblocked before the tunnels 140 are filled with the conductive layer 155.This condition may be primarily satisfied by forming at least twoseparated string selection transistors, i.e., the string selectiontransistors TS1 and TS2, and at least two separated ground selectiontransistors, i.e., the ground selection transistors TG1 and TG2, asdescribed above with reference to FIG. 3. It is possible to control thegate lengths of a string selection gate electrode 166, a control gateelectrode 164, and a ground selection gate electrode 166 illustrated inFIG. 11 by controlling the widths of the second holes 135 and thetunnels 140 as described above with reference to FIG. 3.

Referring to FIG. 11, the conductive layer 155 of FIG. 10 that isexposed via the second holes 135 may be selectively etched in order toform the ground selection gate electrodes 162, the control gateelectrodes 164, and the string selection gate electrodes 166.

Referring to FIG. 12, the string selection gate electrodes 166 may becommonly connected to a first string selection line SSL1 via contactplugs 174. The control gate electrodes 164 may be connected to wordlines WL0, WL1, through WLn−1, and WLn via contact plugs 172. The groundselection gate electrodes 162 may be connected to a first groundselection line GSL1 via contact plugs 170.

FIG. 13 is an embodiment of a circuit diagram of a non-volatile memorydevice according to another aspect of the inventive concept. FIG. 14 isa schematic cross-sectional view of the non-volatile memory device ofFIG. 13, taken from a direction of bit lines. The non-volatile memorydevice illustrated in FIGS. 13 and 14 is a modified example of thenon-volatile memory devices illustrated in FIGS. 1 to 4, according toaspects of the inventive concept. Thus, the operations orcharacteristics of constitutional elements that are the same as those ofFIGS. 1 to 4 will not be described again here.

Referring to FIG. 13, at least one pair of first and second stringselection transistors TS1 and TS2 may be arranged adjacent to a firstend of a plurality of memory cells MC. A first string selection lineSSL1 may be coupled to the first string selection transistors TS1, and asecond string selection line SSL2 may be coupled to the second stringselection transistors TS2. Thus, unlike the non-volatile memory deviceillustrated in FIG. 1, the first and second selection transistors TS1and TS2 may be disposed apart from each other and may operateindividually in this embodiment.

At least one pair of first and second ground selection transistors TG1and TG2 may be arranged adjacent to a second end of the plurality ofmemory cells MC, where the second end is opposite to the first end. Afirst ground selection line GSL1 may be coupled to the first groundselection transistors TG1 and a second ground selection line GSL2 may becoupled to the second ground selection transistors TG2. Thus, unlike thenon-volatile memory device illustrated in FIG. 1, the first and secondground selection transistors TG1 and TG2 may be disposed apart from eachother and may operate individually. In a modified example of the currentembodiment, the first and second ground selection transistors TG1 andTG2 may be connected commonly to one ground selection line GSL, asillustrated in FIG. 1.

An embodiment of a method of operating a non-volatile memory deviceaccording to aspects of the inventive concept will now be described withreference to FIG. 13.

For example, for a program operation of the non-volatile memory device,0V or an operational voltage may be applied to a bit line BL, and 0 Vmay be applied to a common source line CSL. When 0 V is applied to thebit line BL, a NAND string NS is selected to be programmed, but when anoperational voltage is applied to the bit line BL, the NAND string NS isprevented from being programmed due to channel boosting.

A program voltage may be applied to a memory cell MC selected from amongthe plurality of memory cells MC in the NAND string and a pass voltagemay be applied to the remaining memory cells MC in the NAND string. Thepass voltage may be less than the program voltage and be greater than athreshold voltage of the plurality of memory cells MC. The programvoltage may be determined such that electric charges are injected intothe plurality of memory cells MC by F-N tunneling.

An ‘OFF’ voltage may be applied to the first and second ground selectionlines GSL1 and GSL2 of the NAND string. A first voltage may be appliedto the second string selection line SSL2 closest to the plurality ofmemory cells MC and a second voltage may be applied to the first stringselection line SSL2 closest to the bit line BL. The second voltage maybe determined to be as low as possible in order to turn on the firststring selection transistor TS1 while reducing the ‘OFF’ currentthereof. For example, the second voltage may be equal to or greater thana threshold voltage of the first string selection transistor TS1. Thatis, the second voltage may be equal to, for example, the operationvoltage.

The first voltage may be determined such that the difference betweenvoltages of the second string selection transistor TS2 and a memory cellMC adjacent to the second string selection transistor TS2 is minimized.For example, the first voltage may be substantially equal to the passvoltage. As described above, the first voltage may be determined to begreater than the second voltage and to minimize the difference betweenthe first voltage and the pass voltage, thereby preventing channelboosting efficiency from decreasing due to a leakage current flowingthrough the second string selection transistor TS2 adjacent to theplurality of memory cells MC.

Accordingly, in the method of operating a non-volatile memory deviceaccording to the current embodiment, the first and second stringselection transistors TS1 and TS2 are disposed apart from each other andoperate individually, and thus, both an ‘OFF’ current and a leakagecurrent can be reduced.

Referring to FIG. 14, a plurality of string selection gate electrodes166 may be connected to the first and second string selection line SSL1and SSL2 via a plurality of contact plugs 174, respectively. A pluralityof ground selection gate electrodes 162 may be connected to the firstand second ground selection lines GSL1 and GSL2 through a plurality ofcontact plugs 170, respectively.

FIG. 15 is an embodiment of a schematic block diagram of a non-volatilememory system according to another aspect of the inventive concept.Referring to FIG. 15, a NAND cell array 250 may be combined with a corecircuit unit 270. For example, the NAND cell array 250 may include anon-volatile memory as described above. The core circuit unit 270 mayinclude a control logic unit 271, a row decoder 272, a column decoder273, a sense amplifier 274, and/or a page buffer 275.

The control logic unit 271 may communicate with the row decoder 272, thecolumn decoder 273 and/or the page buffer 275. The row decoder 272 maycommunicate with the NAND cell array 250 having a stacked structure viastring selection lines SSL, word lines WL and/or ground selection linesGSL. The column decoder 273 may communicate with the NAND cell array 250via bit lines BL. The sense amplifier 274 may be electrically connectedto the column decoder 273 when it receives a signal from the NAND cellarray 250 and may be electrically disconnected from the column decoder273 when it transmits a signal to the NAND cell array 250.

For example, the control logic unit 271 may transmit a row addresssignal to the row decoder 272, and the row decoder 272 may decode therow address signal and transmit the decoded signal to the NAND cellarray 250 via the string selection lines SSL, the word lines WL, and theground selection lines GSL. The control logic unit 271 may transmit acolumn address signal to the column decoder 273 or the page buffer 275,and the column decoder 273 may decode this signal and transmit thedecoded signal to the NAND cell array 250 via the bit lines BL. A signaloutput from the NAND cell array 250 having the stacked structure may bedelivered to the sense amplifier 274 via the column decoder 273 and beamplified by the sense amplifier 274, and the result of amplificationmay be delivered to the control logic unit 271 via the page buffer 275.

FIG. 16 is an embodiment of a schematic block diagram of a memory card400 according to an aspect of the inventive concept. Referring to FIG.16, the memory card 400 may include a controller 410 and a memory 420 ina housing 430. The controller 410 and the memory 420 may exchange anelectrical signal with each other. For example, data may be exchangedbetween the memory 420 and the controller 410 in response to a commandgiven from the controller 410. Thus, the memory card 400 may store datain the memory 420 or may output data stored in the memory 420 to theoutside.

For example, the memory 420 may include a non-volatile memory device asdescribed above. The memory card 400 may be used as a data storagemedium in various portable apparatuses. For example, the memory card 400may include a multi-media card (MMC) or a secure digital (SD) card.

FIG. 17 is a schematic block diagram of an electronic system 500according to an embodiment of the inventive concept. Referring to FIG.17, the electronic system 500 may include a processor 510, a memory chip520, and an input/output device 530, and data communication may beestablished among the processor 510, the memory chip 520 and theinput/output device 530 via a bus 540. The processor 510 may execute aprogram and control the electronic system 500. The input/output device530 may be used in order to input data to or output data from theelectronic system 500. The electronic system 500 may be connected to anexternal device, e.g., a personal computer (PC) or a network, in orderto exchange data with the external device via the input/output device530. The memory 520 may store code and data for operating the processor510. For example, the memory 420 may include a non-volatile memorydevice as described above.

For example, the electronic system 500 may constitute various electroniccontrol apparatuses that access and/or rely on information in the memory520, and/or that store information in the memory 520. For example, theelectronic system 500 may be used in mobile phones, MP3 players,navigation devices, solid state disks (SSDs) or household appliances—toname just a few examples of possible electronic systems.

While the foregoing has described what are considered to be the bestmode and/or other preferred embodiments, it is understood that variousmodifications can be made therein and that the inventive concept may beimplemented in various forms and embodiments, and that they may beapplied in numerous applications, only some of which have been describedherein. It is intended by the following claims to claim that which isliterally described and all equivalents thereto, including allmodifications and variations that fall within the spirit and scope ofthe following claims.

What is claimed is:
 1. A semiconductor device comprising: a substrate; afirst ground select electrode on the substrate; a second ground selectelectrode on the first ground select electrode; a plurality of cellelectrodes on the second ground select electrode; a first string selectelectrode on the plurality of cell electrodes; a second string selectelectrode on the first string select electrode; a string selection lineconnected to the first string select electrode with a first stringselect contact plug and connected to the second string select electrodewith a second string select contact plug; and a ground selection lineconnected to the first ground select electrode with a first groundselect contact plug and connected to the second ground select electrodewith a second ground select contact plug, wherein a vertical length ofthe first string select contact plug is different from a vertical lengthof the second string select contact plug, and a vertical length of thefirst ground select contact plug is different from a vertical length ofthe second ground select contact plug.
 2. The semiconductor device ofclaim 1, wherein the vertical length of the first string select contactplug is greater than the vertical length of the second string selectcontact plug.
 3. The semiconductor device of claim 1, wherein thevertical length of the first ground select contact plug is greater thanthe vertical length of the second ground select contact plug.
 4. Thesemiconductor device of claim 1, wherein an end of the first groundselect electrode comprises an extended portion that extends beyond anend of the second ground select electrode such that the first groundselect contact plug is on the extended portion of the first groundselect electrode.
 5. The semiconductor device of claim 1, wherein an endof the first string select electrode comprises an extended portion thatextends beyond an end of the second string select electrode such thatthe first string select contact plug is on the extended portion of thefirst string select electrode.
 6. The semiconductor device of claim 1,wherein the string selection line and the ground selection line areabove a top surface of the second string select electrode.
 7. Thesemiconductor device of claim 6, wherein the plurality of cellelectrodes are stacked in a first direction, and the string selectionline and the ground selection line are substantially coplanar in asecond direction that is substantially perpendicular with respect to thefirst direction.
 8. The semiconductor device of claim 1, wherein thesemiconductor device comprises a vertical NAND flash memory.